Systems and methods for providing aircraft heading information

ABSTRACT

Systems and methods for providing aircraft heading information are provided. In one embodiment, an attitude heading reference device comprises: at least one interface for receiving heading information from one or more IRUs; at least one set of gyroscopes and accelerometers; a memory device for storing data representing heading information received via the at least one interface; and a heading calculator coupled to the at least one interface, the at least one set of gyroscopes and accelerometers, and the memory device. The heading calculator generating a heading output signal based on heading information when reliable heading information is received over the at least one interface; the heading calculator generating the heading output signal based on data from the memory device regarding previously reliable heading information and an output of the at least one set of gyroscopes and accelerometers when reliable heading information is not received over the at least one interface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/695,502, filed on Jan. 28, 2010 (pending), the disclosure of which isincorporated herein by reference.

BACKGROUND

One very important piece of information that aircraft pilots must beaware of is the heading of the aircraft they are flying. In the UnitedStates, regulatory agencies quantify this need by requiring that theprobability of a pilot losing all heading information is less than 10⁻⁹per hour. The primary means for obtaining heading information in anairliner or large business jet is typically an inertial reference system(IRS) utilizing gyrocompassing and the rotation of the Earth todetermine what direction an aircraft is heading. It is typical that anaircraft's inertial reference system employ 2 or 3 inertial referenceunits (IRUs) to provide a degree of redundancy to satisfy the 10⁻⁹ perhour regulation. One problem associate with redundancy is that a bank ofidentical IRUs would be susceptible to a common mode failure. That is, alatent defect present in each of the identical IRUs may cause them toall fail under the same conditions at the same time. To mitigateoccurrences of a common mode failure that eliminates all headinginformation, a pilot is typically provided with at least one dissimilarsource of heading information. Traditionally, a magnetic compass hasbeen utilized by pilots to satisfy the need for a dissimilar source ofheading information. However, modern aircraft are increasingly beingcontrolled via electrical/electronic systems and devices that producefields that interfere with the ability of magnetic sensors to measurethe Earth's magnetic field. That is, the increasingly prevalent use ofelectronics on aircraft make it increasingly difficult to find a placeon the aircraft where a magnetic sensor can be installed withoutinterference from magnetic fields generated by other aircraft systems.

For the reasons stated above and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the specification, there is a need in the art for improvedsystems and methods for providing aircraft heading information.

SUMMARY

The Embodiments of the present invention provide methods and systems forproviding aircraft heading information and will be understood by readingand studying the following specification.

Systems and methods for providing aircraft heading information areprovided. In one embodiment, an attitude heading reference devicecomprises: at least one interface for receiving heading information fromone or more IRUs; at least one set of gyroscopes and accelerometers; amemory device for storing data representing heading information receivedvia the at least one interface; and a heading calculator coupled to theat least one interface, the at least one set of gyroscopes andaccelerometers, and the memory device. The heading calculator generatinga heading output signal based on heading information when reliableheading information is received over the at least one interface; theheading calculator generating the heading output signal based on datafrom the memory device regarding previously reliable heading informationand an output of the at least one set of gyroscopes and accelerometerswhen reliable heading information is not received over the at least oneinterface.

DRAWINGS

Embodiments of the present invention can be more easily understood andfurther advantages and uses thereof more readily apparent, whenconsidered in view of the description of the preferred embodiments andthe following figures in which:

FIG. 1 is a block diagram illustrating an inertial system and displaysystem of one embodiment of the present invention;

FIG. 2 is a block diagram illustrating an inertial system of oneembodiment of the present invention; and

FIG. 3 is a flow chart illustrating a method of one embodiment of thepresent invention.

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize features relevant to thepresent invention. Reference characters denote like elements throughoutfigures and text.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of specific illustrative embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and that logical,mechanical and electrical changes may be made without departing from thescope of the present invention. The following detailed description is,therefore, not to be taken in a limiting sense.

Embodiments of the present invention address the need for backup headinginformation by providing an alternate source, dissimilar to theaircraft's installed IRUs, that does not depend on magnetic compasses,magnetometers, or other magnetic sensors to provide accurate headinginformation.

FIG. 1 is a block diagram illustrating an inertial system 100 of oneembodiment of the present invention. Inertial system 100 comprises aplurality of inertial reference units (IRUs) 110, an attitude headingreference system (AHRS) 120, and a display system 130. Although threeIRUs 110 are illustrated by the example of FIG. 1, embodiments of thepresent invention are not so limited. As used herein an IRU wouldinclude any inertial device having capabilities to determine headingthrough gyro-compassing. In alternate embodiments, a greater or fewernumber of IRUs 100 may be present. AHRS 120 comprises at least one setof gyroscopes and accelerometers 122, a voter module 124, a memorydevice 126 and a heading calculator 128. Each of the IRUs 110 arecoupled to the AHRS 120 and provide output signals to voting circuit 124that include heading information. IRUs 110 are capable of providing bothTrue Heading and Magnetic Heading signals. Gyro-compassing calculatesTrue Heading and each IRU 110 converts the True Heading into a MagneticHeading using what is referred to in the art as a “Mag Map.” Embodimentsof the present invention may be implemented using either True orMagnetic Heading signals from an IRU 110. Accordingly, as used herein,the terms “heading signal” and “heading information” refer equally totrue heading or magnetic heading information derived fromgyro-compassing measurements. In the embodiment of FIG. 1, the IRUs 110also output validity information that provides an assessment of thedegree to which the heading information is considered trustworthy (forexample, based on built-in tests within each IRU 110 that indicate whenan IRU 110 has failed). For example, in one embodiment an IRU 110 willoutput validity information indicating that the heading informationsignal it provides is trustworthy when the internal tests performed inthe IRU 110 do not have any failures that would cause the heading signalto be inaccurate. When the internal tests within an IRU 110 detect sucha failure, it will output the heading information along with validityinformation indicating that the heading information falls outside of theaccuracy tolerance criteria, or it may simply stop transmitting anyheading information.

When reliable heading information is being received from at least one ofthe IRUs 110, AHRS 120 operates in a “normal mode” where it generates aheading output signal that is derived from a slaving signal. The slavingsignal is generated from heading information deemed reliable from theIRUs 110. As would be appreciated by one of ordinary skill in the art,the gyroscopes and accelerometers 122 are significantly less precisethan those within the IRUs 110 and are not sufficient to performgyro-compassing. Thus, the slaving signal derived from the output of theIRUs 110 provides AHRS 120 with a reliable heading from which it canmeasure deviations using the gyroscopes and accelerometers 122. Theslaving signal also allows the AHRS 120 to estimate and remove biaserrors in the AHRS gyroscopes 122. Heading calculator 128 can thengenerate heading information that is more accurate than it could begenerated based on its gyroscopes and accelerometers 122 alone. Inbackup mode, when no reliable heading information is being received fromIRUs 110, AHRS 120 enters a directional-gyro (DG) mode of operation. InDG mode, Heading calculator 128 utilizes the last reliable headinginformation it received from the IRUs 110 to convert measurements bygyroscopes and accelerometers 122 into backup heading information.

In operation, in one embodiment, voter module 124 performs the functionof determining the makeup of the slaving signal that AHRS 120 uses innormal node and whether AHRS 120 operates in normal mode or DG mode. Asshown in FIG. 1, the heading and validity information from each of theIRUs 110 is received at voter module 124. Voter module 124 evaluatesthis information to determine what, if any, reliable heading informationis being received from the IRUs 110.

In one embodiment, as long as reliable heading information is availablefrom at least one of the IRUs 110, voter module 124 produces a headingslaving signal using the reliable heading information. For example, whenreliable heading information is available from two or more of the IRUs110, voter module 124 averages the reliable heading information toproduce the slaving signal. When reliable heading information isavailable from only one of the IRUs 110, then the slaving signal isproduced from that single reliable heading information signal.

When voter module 124 determines that no reliable heading information isbeing provided by the IRUs 110, then the voter module 124 instructs AHRS120 to shift into DG mode. In one embodiment, voter module 124 instructsAHRS 120 to shift into DG mode via the slaving signal. For example, inone embodiment, when voter module 124 determines that no reliableheading information is being provided by the IRUs 110, the slavingsignal is turned off and AHRS 120 shifts to DG mode based on the loss ofthe slaving signal. In another embodiment, voter module 124 generates aspecific signal or message to switch AHRS 120 to DG mode.

In one embodiment, voter module 124 utilizes the validity informationsignal produced by the IRUs 110 to make the determination as to whetherheading information from each respective IRU 110 will be used togenerate the slaving signal. In one alternate embodiment, either insteadof or in addition to utilizing the validity information, voter module124 independently determines reliability of heading information from theIRUs 110 based on the degree to which the heading information from eachof the IRUs 110 agree. For example, in one embodiment, when the headinginformation from one of the IRUs 110 deviates beyond a predeterminedthreshold (as compared to the heading information from the other IRUs)then that heading information will not be used for generating theslaving signal. If the heading information from two remaining IRUs 110begins to deviate beyond the threshold, then voting circuit will causeAHRS 120 to shift to DG mode.

In another alternate embodiment, instead of (or in addition to) having avoter module 124 vote the heading information signals from the IRUs 110,the AHRS 120 monitors for failures of the IRUs 110 by comparing headinginformation from the IRUs 110 against heading information measured bythe AHRS 120 accelerometers and gyroscopes 122. If the Headingcalculator 128 detects an inconsistency, such as a significant abruptdifference between the IRU 110 and AHRS 120 heading, the Headingcalculator 128 could conclude that IRU 110 heading has failed, andswitch to DG mode.

As shown in FIG. 1, in one embodiment, AHRS 120 includes a memory device126 for storing reliable heading information received from the IRUs 110.In one embodiment, memory device 126 stores the last known reliableheading information as represented by the slaving signal. When AHRS 120shifts into DG mode, the last known reliable heading information isretrieved by heading calculator 128 from memory device 126 forgenerating the backup heading information described above. Although FIG.1 indicates voter module 124 and memory device 126 being integratedwithin AHRS 120, one of ordinary skill in the art upon reading thisspecification would appreciate that other embodiments include one orboth of these devices implemented as discreet components within system100.

In one embodiment, heading information generated by AHRS 120, whether innormal mode or DG mode, is provided to an aircraft's flight crew viadisplay system 130. In one embodiment, display system 130 include aprimary display device 132 for providing heading information to theCaptain from one of the IRUs 110, a primary display device 133 forproviding for providing heading information to the first officer from adifferent one of the IRUs 110, and a standby display device 134 forproviding backup heading information to both the Captain and the firstofficer from the AHRS 120. In one embodiment, AHRS 120 indicates to theflight crew via display system 130 whether it is operating in normal orDG mode. Display system 130 in turn notifies the flight crew via displaydevices 132, 133 whether the heading information provided by AHRS 120 isnormal mode heading information or backup mode heading information.

In addition to the 10⁻⁹ availability requirement established byregulation, their may also exists integrity requirements established byregulation. For example, a regulation may require that probability of aflight crew is given an incorrect heading solution is less than 10⁻⁷perhour. As described in the previous section, the AHRS 120 will revertto DG mode for situations where reliable heading information is notavailable from the IRUs 110. However, the heading information providedwhile in DG mode will drift over time from the aircraft' actual headingbecause the gyroscopes and accelerometers 122 will likely be lessprecise than those within the IRUs 110 and thus the AHRS 120 headinginformation provided in DG mode is susceptible to drifting over time.For this reason, in one alternate embodiment, after a predeterminedperiod of time in DG mode, Heading calculator 128 will switch to a trackangle mode based on information provided by a Global NavigationSatellite System (GNSS) receiver such as Global Positioning System (GPS)receiver 114. As would be appreciated by one of ordinary skill in theart upon reading this specification, information from a GNSS receivercannot be used to determine which direction an aircraft is pointing, butcan be used to calculate the aircraft's track angle which indicates thedirection the aircraft is moving. In the absence of wind, heading andtracking should be the same, but in the presence of wind currents, theaircrafts flight path will deviate from the direction it pointing.

Table 1, below, illustrates the AHRS 120 operating mode for examplescenarios of one embodiment of the present invention.

TABLE 1 SCE- NARIO IRU MAG HEADING AHRS MAG HEADING # STATUS SLAVING/DGMODE 1 At least two IRUs 110 have The AHRS 120 slaves its valid headingsignals that heading output to the average compare within a reasonableof the valid heading signals. threshold. 2 Heading signals from all IRUsAHRS 120 reverts to DG 110 fail simultaneously (For Mode. example, thismight happen due to a common-mode failure in the IRUs 110). 3 Headingsignals from all but one AHRS 120 slaves its heading IRU 110 fail.output to the heading signal from the remaining IRU 110. This providesheading avail- ability for the scenario where the airplane is dispatchedwith only two operational IRUs 110 and one of the IRUs 110 failsin-flight. 4 Scenario #3, then the good IRU AHRS 120 reverts to DG 110stops transmitting or goes Mode. invalid. 5 At least two of the IRUs 110AHRS 120 reverts to DG have valid heading signals that Mode. comparewithin a reasonable threshold, then the two heading signals begin tomiscompare even though the IRUs 110 report them as valid (i.e., afailure undetected by the IRUs 110).

As illustrated in FIG. 2, in one embodiment, an inertial system maycomprise two AHRS units that function independently from each other.FIG. 2 illustrates an inertial system 200 comprising a plurality ofinertial reference units (IRUs) 210, and a pair of attitude headingreference system (AHRS) 220-1 and 220-2, each coupled to a displaysystem 230. For the particular embodiment illustrated, each of the AHRS220-1 and 220-2 receives heading and validity information from all threeof the IRUs 210 Each of the AHRS 220-1 and 220-2 independently provideheading information to display system 230, each operating in the samemanner as described with respect to AHRS 120 in FIG. 1. For example, forthe example scenarios described in Table 1, each of the AHRS 220-1 and220-2 would switch between normal and backup (DG) mode in the samemanner as indicated for AHRS 120.

FIG. 3 is a flow chart illustrating a method of one embodiment of thepresent invention for providing standby heading information. The methodbegins at 310 with determining a reliability of inertial reference unitgenerated heading information signals. In one embodiment determiningwhen reliable heading information is received from at least one inertialreference unit is based on validity information provided by the inertialreference units. In one embodiment, a voter module evaluates what, ifany, reliable heading information is being received from the IRUs andproduces a slaving signal. In one embodiment, as long as reliableheading information is available from at least one IRU, the voter moduleproduces a slaving signal using the reliable heading information. Forexample, when reliable heading information is available from two or moreIRUs, the method includes the voter module averaging the reliableheading information to produce the slaving signal. When reliable headinginformation is available from only one IRU, then the slaving signal isproduced from that single reliable heading information signal. In oneembodiment, the method includes monitoring for failures of the IRUs bycomparing any heading information from the IRUs against heading changesmeasured by AHRS accelerometers and gyroscopes. When the method at 310,in one embodiment, identifies a significant abrupt difference betweenthe IRU and AHRS heading information, the method concludes that that IRUheading information is not available and switches the AHRS to DG mode.

When reliable heading information is received from at least two inertialreference units (determined at 315), the method proceeds to 320 withstoring data representing the reliable heading information in a memorydevice and to 330 with generating a heading output signal using thereliable heading information as a slaving signal. In one embodiment, aslong as reliable heading information is available from at least one ofthe IRUs, a voter module produces a slaving signal using the reliableheading information. For example, when reliable heading information isavailable from two or more of the IRUs, the voter module averages thereliable heading information to produce the slaving signal. In onealternate embodiment, when reliable heading information is availablefrom only one of the IRUs, then the slaving signal is produced from thatsingle reliable heading information signal. The slaving signal derivedfrom the output of the IRUs provides the AHRS with a reliable headingfrom which it can measure deviations using its own gyroscopes andaccelerometers. The AHRS can then generate heading information that ismore accurate than it could be generated based on its gyroscopes andaccelerometers alone.

When reliable heading information is not received from at least twoinertial reference units (determined at 315), the method proceeds to 340with reading the data representing the reliable heading information fromthe memory device and to 350 with generating the heading output signalfrom that data along with the output provided by the AHRS's set ofgyroscopes and accelerometers. When this occurs, the AHRS is switched tooperate in a directional-gyro (DG) mode. In DG mode, the AHRS utilizesthe last reliable heading information it received from the IRUs (whichis stored in the memory device) to convert measurements by gyroscopesand accelerometers into backup heading information. The heading outputsignal may then be used for applications such as, but not limited to,displaying heading information to a flight crew and providing headinginformation to a flight control computer. In one embodiment, if reliableheading information is lost but later returns, the method will switchthe back to normal operation from the DG mode operation.

Several means are available to implement the systems and methods of thecurrent invention as discussed in this specification. These meansinclude, but are not limited to, digital computer systems,microprocessors, general purpose computers, programmable controllers andfield programmable gate arrays (FPGAs). For example, in one embodiment,computer system 120 is implemented by an FPGA or an ASIC, or an embeddedprocessor. Therefore other embodiments of the present invention areprogram instructions resident on computer readable media which whenimplemented by such means enable them to implement embodiments of thepresent invention. Computer readable media include any form of aphysical computer memory device. Examples of such a physical computermemory device include, but is not limited to, punch cards, magneticdisks or tapes, optical data storage system, flash read only memory(ROM), non-volatile ROM, programmable ROM (PROM), erasable-programmableROM (E-PROM), random access memory (RAM), or any other form ofpermanent, semi-permanent, or temporary memory storage system or device.Program instructions include, but are not limited to computer-executableinstructions executed by computer system processors and hardwaredescription languages such as Very High Speed Integrated Circuit (VHSIC)Hardware Description Language (VHDL).

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiment shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. An inertial system, the system comprising: one ormore inertial reference units (IRUs) each providing a headinginformation signal; and an attitude heading reference system (AHRS)receiving the heading information signal from each of the one or moreIRUs, the AHRS including at least one set of gyroscopes andaccelerometers, a memory device and a heading calculator; the headingcalculator configured to receive a slaving signal that is generatedbased on the heading information signal from at least one of the one ormore IRUs, the heading calculator further configured to receive a signalfrom the at least one set of gyroscopes and accelerometers; wherein,when reliable heading information is being received from at least one ofthe one or more IRUs, the AHRS operates in a normal mode where theheading calculator generates a heading output signal from the slavingsignal; wherein, when reliable heading information is not received fromat least one of the one or more IRUs, the AHRS switches to adirectional-gyro (DG) mode of operation where the heading calculatorgenerates the heading output signal from an output of the at least oneset of gyroscopes and accelerometers and information stored in thememory device regarding previously reliable heading information.
 2. Thesystem of claim 1, further comprising a Global Navigation SatelliteSystem (GNSS) receiver, wherein after the AHRS operates a predeterminedperiod of time in DG mode, the heading calculator will switch to a trackangle mode based on information provided by the GNSS receiver.
 3. Thesystem of claim 1, further comprising a voter module, wherein the votermodule utilizes validity information signals produced by at least one ofthe one or more IRUs to generate the slaving signal and switch the AHRSto DG mode.
 4. The system of claim 3, wherein the voter module is adevice external to the AHRS and coupled between the plurality of IRUsand the AHRS.
 5. The system of claim 1, further comprising a votermodule for generating the slaving signal, wherein the voter moduledetermines a reliability of heading information provided from the one ormore IRUs based on relative agreement of heading information between atleast two IRUs.
 6. The system of claim 1, wherein the one or more IRUseach further output validity information that provides a reliabilityassessment of the heading information.
 7. The system of claim 1, furthercomprising a display system that displays the heading output generatedby the heading calculator.
 8. The system of claim 1, wherein the AHRSmonitors for failures of the one or more IRUs by comparing headinginformation from the one or more IRUs against heading informationmeasured by the at least one set of accelerometers and gyroscopes;wherein when the AHRS detects an inconsistency between the headinginformation from the one or more IRUs and heading information measuredby the at least one set of accelerometers and gyroscopes, the AHRSswitches to the DG mode.
 9. A method for providing standby headinginformation, the method comprising: determining a reliability ofinertial reference unit generated heading information signals; whenreliable heading information is received from at least one inertialreference unit, storing data representing the reliable headinginformation in a memory device and generating a heading output signalfrom the reliable heading information; when reliable heading informationis not received from at least one inertial reference unit, reading thedata representing the reliable heading information from the memorydevice and generating the heading output signal from the data and anoutput of at least one set of gyroscopes and accelerometers of anattitude heading reference system (AHRS) operating in a directional-gyro(DG) mode.
 10. The method of claim 9, further comprising generating aslaving signal from the reliable heading information.
 11. The method ofclaim 9, further comprising determining when reliable headinginformation is received from at least one inertial reference unit basedon validity information provided by the at least one inertial referenceunit.
 12. The method of claim 9, wherein when determining a reliabilityof inertial reference unit generated heading information signalsdetermines that reliable heading information is not being received, themethod further comprises switching the AHRS to the DG mode.